The present invention relates to a method and apparatus for automatically controlling the length of sheets cut from a moving web of corrugated material.
In particular, the invention relates to a method and apaparatus for dynamically determining coarse errors in sheet length which are rapidly corrected at high speed by a positioning motor coupled to a variable speed transmission. Fine errors in sheet length are corrected by the positioning motor at a slower speed. The non-linear relationship between positioning motor shaft position and sheet length correction is automatically compensated for without sensing rotation of the positioning motor shaft.
In the manufacture of corrugated board, a moving web of corrugated material is repetitively severed by rotating cutting knives. The speed of the web and the speed of the cutting knives ultimately determines the length of the sheets being cut. A variable speed transmission drives the cutting knives at a fraction of the web speed. A positioning motor adjusts the variable speed transmission to correct for errors in sheet length.
It is desirable to separate sheet length errors into two groups, coarse and fine, depending upon the lengths of the sheets being cut. Thus, a fixed error in sheet length may be viewed as coarse for a short sheet but fine for a long sheet. If a two speed positioning motor is employed, the coarse error can be corrected rapidly at a high speed while the fine error can be corrected at a somewhat slower speed. In this manner, sheet length error correction can be effected accurately in a minimal time period.
In correcting for a fixed sheet length error, the number of turns of the positioning motor shaft varies non-linearly with the length of the sheet being cut. Thus, in correcting the same error, the positioning motor shaft must be driven a greater number of turns for shorter sheets than for longer sheets. This is due primarily to the non-linear characteristic of the variable speed transmission.
The non-linear relationship between rotative displacement of the positioning motor shaft and resulting correction in sheet length error can be compensated for by regulating the charging time of a capacitor through a series of adjustable potentiometers to effect incremental changes in sheet length. This technique is relatively slow and inaccurate. An alternative method is to actually sense the rotative displacement of the positioning motor shaft and process this information through a feedback loop in the cut-off control. This involves the use of expensive auxiliary detection equipment such as optical shaft encoders and the like. In such a system, the speed of correction of the error is fixed by the pulse repetition frequency of the pulses generated by the encoder.
Prior art cut-off controls which compensate for the non-linear characteristic of the variable speed transmission are disclosed in U.S. Pat. Nos. 3,324,751, 3,411,388, 3,668,957 and 3,608,411.
An advantage of the present invention is that coarse errors in sheet length can be rapidly detected by dynamically operating a position register indicative of the rotative displacement of the positioning motor shaft.
An another advantage of the invention is that the non-linear relationship between sheet length correction and turns of the positioning motor shaft is automatically compensated for in correcting for coarse and fine errors in sheet length without sensing the actual displacement of the positioning motor shaft.
A further advantage of the invention is that coarse errors are corrected rapidly at a high speed while fine errors are corrected at a slower speed.
A still further advantage of the invention is that error detection and correction is accomplished digitally, over a minimal time interval and with increased accuracy and reliability.
Other advantages will appear hereinafter.